PayPal occasionally updates the way the checkout systems work and we try to make sure that eCart get's updated to work with the changes. I seem to recall an update that was released for eCart some time ago that accounts for this success with warning response.
To update the current checkout you should start with a copy of the checkout pages and try reapplying the local checkout server behavior with the latest release of eCart. You should then run some test transactions through the sandbox to ensure that the checkout is functioning properly.